This invention pertains generally to the field of electrolytic capacitors ("elcaps") and specifically to the composition of the electrolyte used in elcaps.
The basic structure of an electrolytic capacitor is known. In an elementary form, a cylindrical container or can houses a "roll" comprised of a cathode foil, a first paper, an anode foil and a second paper. The first and second papers are porous separators which hold the fill electrolyte by capillary action. The papers and films are rolled onto the cathode foil; thus, the first and second papers separate the anode and cathode foils in the roll, while an end portion of the second paper is also interposed between the anode and the can. See FIG. 1. A tab connected to each of the anode and the cathode foils and projecting through the top of the can provide the electrodes.
An insulating oxide film is usually formed on the side of the anode facing the cathode. The electrolytic solution of the elcap is a conducting solution. Thus, the solution immediately adjacent the anode (on the cathode side of the anode) actually performs the function of the cathode. The oxide film forms a dielectric barrier between the anode and the electrolyte.
Accordingly, the characteristics of the elcap are determined in large part by the composition of the electrolyte. The conducting electrolytic solution is generally comprised of acid and salt. Prior efforts to improve the performance of elcaps, including reduction of the ESR, increasing the sparking voltage level, etc. have been directed at the composition of the electrolyte, among other things. Another important feature of the elcap, and thus the electrolyte, is its ability to perform over an acceptable range of temperatures. For example, an electrolyte that precipitates into solid particles at the low end of the operating range (such as room temperature), is usually unacceptable. Such characteristics can be related to the pH range of the solution; in many prior elcaps, the solution had to remain slightly basic in order to hold the acids together in the particular solvent.
In particular, maintaining the soluability of heavy dicarboxylic acid in glycol-like solutions containing borates has been difficult. The addition of borates to a composition having heavy dicarboxylic acids, even in a somewhat basic solution, results in the heavy acid separating from solution as a separate phase when it is standing at room temperature.